Preheat the oven to 350°F (175°C).
Grease a muffin tin with cooking spray or oil.
In a large bowl, combine the spinach, bell peppers, onion, salt, pepper, and garlic powder.
Add the egg whites to the vegetable mixture and stir until well combined.
Pour the mixture into the greased muffin tin, filling each cup about 3/4 full.
Bake in the preheated oven for 18-20 minutes or until the muffins are set and lightly golden.
Allow the muffins to cool slightly in the pan before transferring to a wire rack.
Serve warm or store in the refrigerator for later.